1. Field of the Invention
The present invention relates to a carbon electrode for melting quartz glass and a method of fabricating the same, and in particular to such a carbon electrode for melting quartz enabling to check occurrence of bubbles in the molten quartz glass and a method of fabricating the same.
2. Description of the Related Art
For fabricating quartz glass, for example, a quartz glass crucible for lifting silicon single crystal, an operation has been carried out by using an apparatus 21 for fabricating a quartz glass crucible as shown in FIG. 3. A mold 22 for forming a crucible of a fabricating apparatus 21 is composed of an inside member 23 made of porous carbon capable of reducing pressure and a holder 24 and is rotatably supported by a rotating shaft 25. At an upper part opposite to the inside member 23, there are provided three pieces of carbon electrodes 27 for melting quartz glass by arc discharge.
Accordingly, for fabricating the quartz glass crucible by using the apparatus 21, the rotating shaft 25 is rotated to rotate the crucible forming mold 22 at high speed. Into the mold 22, silica powder of high purity is supplied from the upper part through a supply pipe. The supplied silica powder is forcibly pressed by centrifugal force to the inside member 23 of the crucible forming mold 22, and a formed body 26 of a crucible shape is obtained.
Subsequently, the interior of the inside member 23 is effected by reducing pressure, and the carbon electrodes 27 are electrically conduced to heat the inside of the formed body 26 so as to form a melted layer of quartz glass on an inner surface of the formed body 26.
In the course of melting the quartz glass by the arc discharge and fabricating the quartz glass crucible, carbon particles often fall off from the carbon electrodes 27 into the quartz glass crucible under molten condition. Carbon particles dropping from the carbon electrodes 27 are oxidized and consumed in the quartz glass crucible during melting, and bubbles are generated in the molten quartz glass (transparent layer). In particular, if silicon single crystal is lifted by using the quartz glass crucible having bubbles in the quartz glass, bubbles existing in the transparent layer formed in the vicinity of the inner surface (in particular, the transparent layer) of the quartz glass crucible are expanded into large bubbles, and the bubbles are mixed into the melting silicon concurrently with melting of the transparent layer, and bad influences are given to growth of silicon single crystal and lower single crystallizing yield of silicon single crystal. In addition, the life of carbon electrode is shortened by the consumption of the carbon electrode 27 going with the arc discharge.
Accordingly, it has been demanded to realize such a carbon electrode of less consumption and a long useful life, and a method of fabricating the same which prevents particles from dropping from carbon electrodes into the molten quartz glass, checks occurrence of bubbles in the molten quartz glass, and avoids lowering of a single crystallizing yield by bubbles existing in the transparent layer also when lifting silicon single crystal by means of the quartz glass crucible made by using this carbon electrode.
The invention has been established in view of the above mentioned circumstances, and it is an object of the invention to provide a carbon electrode of less consumption and a long useful life, and a method of fabricating the same which prevents carbon particles from falling from the carbon electrodes into the liquid molten quartz glass, checks occurrence of bubbles in the quartz glass, and avoids lowering of a single crystallizing yield by bubbles existing in the transparent layer when lifting silicon single crystal by means of the quartz glass crucible made by using this carbon electrode.
A first invention made for accomplishing the above mentioned object is of a subject matter to provide a carbon electrode to be used for melting quartz glass by an arc discharge, characterized in that an electrode base material comprises carbon of bulk density being 1.80 g/cm3 or higher and a three-point bending strength being 35 MPa or higher.
A second invention is of a subject matter characterized in that the electrode base material comprises carbides of a carbonaceous raw material and a binding material, and a maximum particle diameter of the carbon raw material is 150 xcexcm or less, and 90 wt % or more of the carbon raw material has particle diameter of 75 xcexcm or less.
A third invention is of a subject matter characterized in that a secondary particle comprises the above electrode base material composed of the carbonaceous raw material and the binding material, and is an isotropic graphite material combined by carbide of the binding material, a maximum particle diameter of the secondary particle is 500 xcexcm or less, and 50 wt % or more of the secondary particle has particle diameter of 38 to 500 xcexcm.
A fourth invention is of a subject matter characterized in that an anisotropic ratio of specific resistance of the electrode material is 1.1 or less, and an ash content is 5 ppm or less.
A fifth invention is a method of fabricating carbon electrode for melting quartz glass, characterized in that a kneaded material is obtained by heating and kneading the carbonaceous raw material and the binding material, and is pulverized, said carbonaceous raw material having a maximum particle diameter of 150 xcexcm or less, and 90 wt % or more thereof being 75 xcexcm or less of the particle diameter and said binding material having residual carbon of 50% or more, and thereafter the secondary particle is sieved such that the maximum particle diameter is 500 xcexcm or less, and 50 wt % or more is 38 to 500 xcexcm of particle diameter, and is formed into CIP (Cold Isstatic Pressing), followed by, after baking the CIP, processing an isotropic graphite material graphitized at 2900 to 3100xc2x0 C. and carrying out a purifying process on the CIP.
A sixth invention is a method of fabricating a carbon electrode for melting quartz glass, characterized in that a kneaded material is obtained by heating and kneading the carbonaceous raw material and the binding material and is extruded at 130 to 200xc2x0 C., said carbonaceous raw material having a maximum particle diameter of 150 xcexcm or less, and 90 wt % or more thereof being 75 xcexcm or less of the particle diameter and said binding material having residual carbon of 50% or more, and the extruded material is baked and processed into a graphite material graphitized at 2900 to 3100xc2x0 C. and further carried out with a purifying process.
A seventh invention is a method of fabricating carbon electrode for melting quartz glass, characterized in that an isotropic graphite material or a graphite material is impregnated with a thermosetting resin, dried and baked, and then ground on an outer circumference to an extent that the thermosetting resin deposited on a surface is at least removed, and carried out with the above mentioned process.
An eighth invention is a method of fabricating carbon electrode for melting quartz glass, characterized in that a volatile content of the kneaded material is adjusted to be 12 to 15%.
A ninth invention is a method of fabricating carbon electrode for melting quartz glass, characterized in that a carbonaceous raw material is a coal based pitch coke, and a binding material is a coal based tar pitch.
The ash content is measured, by being based on JIS R 7223(1979).